I am confused about what the topic is now. I see the timer as a way to prevent overcharging. The definition of "overcharging" is letting the voltage go too high.
If you have a charger with a max voltage that is not too high, and is not automatic (for shutting down by itself) then the timer will do that. Deciding how long to set the timer for would be from experience with that set-up.
When my AGMs are full at bottomed out amps and then amps go back up with rising temperature but with the voltage still the same from the fixed voltage charger, I don't like that. It is not "overcharging", but it is "going too far" IMO.
I don't understand why Mex says you can just leave that going and it will not hurt the AGM. Perhaps my AGMs should not do that, but not in such good shape. Mex might be talking about AGMs in good shape?
Anyway, a timer set to the right time would catch my AGMs before they heated up too much. This would save me from having to be there when amps bottom out, which could be convenient at times.
I still think a "smart chargers" has its place in the "tool box". You do need to know what it will do and not do.